[Clarification] from/to_stop_id & from/to_trip_id description in tran…

…sfers.txt (#455)

* Clarify from/to_stop_id desc in transfers.txt

* Changed presence for transfers.from/to_stop_id

* Update gtfs/spec/en/reference.md

Co-authored-by: isabelle-dr <[email protected]>

* Update gtfs/spec/en/reference.md

Co-authored-by: isabelle-dr <[email protected]>

* Update gtfs/spec/en/reference.md

Co-authored-by: isabelle-dr <[email protected]>

* Editorial changes on to_trip_id

* Add "location_type=0" for "stop"

---------

Co-authored-by: isabelle-dr <[email protected]>

gtfs/spec/en/reference.md

@@ -633,12 +633,12 @@ For a given ordered pair of arriving trip and departing trip, the transfer with
 
 |  Field Name | Type | Presence | Description |
 |  ------ | ------ | ------ | ------ |
-|  `from_stop_id` | Foreign ID referencing `stops.stop_id` | **Conditionally Required** | Identifies a stop or station where a connection between routes begins. If this field refers to a station, the transfer rule applies to all its child stops. Refering to a station is forbiden for `transfer_types` 4 and 5. |
-|  `to_stop_id` | Foreign ID referencing `stops.stop_id` | **Conditionally Required** | Identifies a stop or station where a connection between routes ends. If this field refers to a station, the transfer rule applies to all child stops. Refering to a station is forbiden for `transfer_types` 4 and 5. |
+|  `from_stop_id` | Foreign ID referencing `stops.stop_id` | **Conditionally Required** | Identifies a stop (`location_type=0`) or a station (`location_type=1`) where a connection between routes begins. If this field refers to a station, the transfer rule applies to all its child stops. It must refer to a stop (`location_type=0`) if `transfer_type` is `4` or `5`.<br><br>Conditionally Required:<br>- **Required** if `transfer_type` is `1`, `2`, or `3`.<br>- Optional if `transfer_type` is `4` or `5`. |
+|  `to_stop_id` | Foreign ID referencing `stops.stop_id` | **Conditionally Required** | Identifies a stop (`location_type=0`) or a station (`location_type=1`) where a connection between routes ends. If this field refers to a station, the transfer rule applies to all child stops. It must refer to a stop (`location_type=0`) if `transfer_type` is 4 or 5.<br><br>Conditionally Required:<br>- **Required** if `transfer_type` is `1`, `2`, or `3`.<br>- Optional if `transfer_type` is `4` or `5`. |
 | `from_route_id`  | Foreign ID referencing `routes.route_id` | Optional | Identifies a route where a connection begins.<br><br>If `from_route_id` is defined, the transfer will apply to the arriving trip on the route for the given `from_stop_id`.<br><br>If both `from_trip_id` and `from_route_id` are defined, the `trip_id` must belong to the `route_id`, and `from_trip_id` will take precedence. |
 | `to_route_id`  | Foreign ID referencing `routes.route_id` | Optional | Identifies a route where a connection ends.<br><br>If `to_route_id` is defined, the transfer will apply to the departing trip on the route for the given `to_stop_id`.<br><br>If both `to_trip_id` and `to_route_id` are defined, the `trip_id` must belong to the `route_id`, and `to_trip_id` will take precedence. |
-| `from_trip_id`  | Foreign ID referencing `trips.trip_id` | **Conditionally Required** | Identifies a trip where a connection between routes begins.<br><br>If `from_trip_id` is defined, the transfer will apply to the arriving trip for the given `from_stop_id`.<br><br>If both `from_trip_id` and `from_route_id` are defined, the `trip_id` must belong to the `route_id`, and `from_trip_id` will take precedence. REQUIRED if `transfer_type` is `4` or `5`. |
-| `to_trip_id`  | Foreign ID referencing `trips.trip_id` | **Conditionally Required** | Identifies a trip where a connection between routes ends.<br><br>If `to_trip_id` is defined, the transfer will apply to the departing trip for the given `to_stop_id`.<br><br>If both `to_trip_id` and `to_route_id` are defined, the `trip_id` must belong to the `route_id`, and `to_trip_id` will take precedence. REQUIRED if `transfer_type` is `4` or `5`. |
+| `from_trip_id`  | Foreign ID referencing `trips.trip_id` | **Conditionally Required** | Identifies a trip where a connection between routes begins.<br><br>If `from_trip_id` is defined, the transfer will apply to the arriving trip for the given `from_stop_id`.<br><br>If both `from_trip_id` and `from_route_id` are defined, the `trip_id` must belong to the `route_id`, and `from_trip_id` will take precedence.<br><br>Conditionally Required:<br>- **Required** if `transfer_type` is `4` or `5`. <br>- Optional otherwise. |
+| `to_trip_id`  | Foreign ID referencing `trips.trip_id` | **Conditionally Required** | Identifies a trip where a connection between routes ends.<br><br>If `to_trip_id` is defined, the transfer will apply to the departing trip for the given `to_stop_id`.<br><br>If both `to_trip_id` and `to_route_id` are defined, the `trip_id` must belong to the `route_id`, and `to_trip_id` will take precedence. <br><br>Conditionally Required:<br>- **Required** if `transfer_type` is `4` or `5`. <br>- Optional otherwise. |
 |  `transfer_type` | Enum | **Required** | Indicates the type of connection for the specified (`from_stop_id`, `to_stop_id`) pair. Valid options are:<br><br> `0` or empty - Recommended transfer point between routes.<br>`1` - Timed transfer point between two routes. The departing vehicle is expected to wait for the arriving one and leave sufficient time for a rider to transfer between routes.<br>`2` - Transfer requires a minimum amount of time between arrival and departure to ensure a connection. The time required to transfer is specified by `min_transfer_time`.<br>`3` - Transfers are not possible between routes at the location.<br>`4` - Passengers can transfer from one trip to another by staying onboard the same vehicle (an "in-seat transfer"). More details about this type of transfer [below](#linked-trips).  <br>`5` - In-seat transfers are not allowed between sequential trips. The passenger must alight from the vehicle and re-board. More details about this type of transfer [below](#linked-trips). |
 |  `min_transfer_time` | Non-negative integer | Optional | Amount of time, in seconds, that must be available to permit a transfer between routes at the specified stops. The `min_transfer_time` should be sufficient to permit a typical rider to move between the two stops, including buffer time to allow for schedule variance on each route. |